Tagatose has the natural sweetness that is hardly distinguished from that of sucrose. The physical properties of tagatose are also very similar to those of sucrose. However, the in-taken tagatose is hard to be absorbed in the small intestine, suggesting that it does not affect blood sugar level. Tagatose is also a low caloric sweetener whose calorie is only about 30% of that of sucrose. Tagatose has a prebiotic effect accelerating the proliferation of beneficial lactobacillus through fermenting by rumen microorganisms.
However, this beneficial tagatose is a rare sugar which is not distributed in nature but included in diary products or some plants at a very low concentration. To use tagatose as a low caloric functional sweetener, a novel technique is required to mass-produce tagatose from an inexpensive raw material.
U.S. Pat. No. 5,002,612 and NO 5078796 describe a method for producing D-tagatose, in which lactose or a lactose-containing material is hydrolyzed into galactose and glucose mixture by using lactase and then glucose is eliminated randomly, followed by chemical isomerization of galactose into tagatose.
U.S. Pat. No. 6,057,135 describes a method for producing tagatose comprising the following steps: obtaining galactose and glucose by hydrolyzing cheese whey or milk; isomerizing the obtained galactose by L-arabinose isomerase; and recycling non-converted compounds after the separation of the products and non-converted compounds by chromatography.
The major raw materials which have been used for the production of tagatose so far are milk processing by-products such as lactose or lactose containing materials. In the bioconversion process that uses lactose or lactose containing materials as a starting material, the production of tagatose should be basically performed by two-step reaction (lactose→galactose→tagatose).